Heat index combines air temperature and relative humidity to determine how hot it actually feels to the human body, since high humidity reduces the body's ability to cool itself through sweat evaporation. This heat index calculator computes the perceived temperature based on actual temperature and humidity readings, using the formula employed by the US National Weather Service. Understanding heat index is important for preventing heat exhaustion and heat stroke during hot weather, especially for outdoor workers, athletes, elderly individuals, and young children who are more vulnerable to heat-related illness. When the heat index reaches dangerous levels, even temperatures that seem moderate on a thermometer can pose serious health risks.